1. Field of the Invention
The present invention relates to a technique for providing a power supply circuit for driving a power transistor which is compact.
2. Description of the Related Art
There is a per se known drive circuit for a power transistor according to the prior art which is disclosed, for example, in xe2x80x9cController for Fork Lift using SITxe2x80x9d by Tamotsu Hoda and Toshio Yoshizawa, Toyoda Industries corporation K.K., Proceedings of Electronic Device Study Group, vol. EDD-90-64, pp. 57-64, 1990.
FIGS. 6A through 6C are figures showing the structure of a prior art drive circuit for a power transistor. FIG. 6A is a figure showing the overall structure of this drive circuit. With such a prior art type drive circuit for a power transistor, optimization of the base electrical current from the low electrical current region all through to the high electrical current region is implemented as a measure in order not to over-saturate the power transistor 1. In other words, the electrical current which is supplied to the base of the power transistor 1 is controlled by a drive circuit which employs a comparator or an ON gate circuit while an operation control signal which controls the operation and stopping of the load is ON, so that, if the power transistor 1 becomes over-saturated, the supply of electrical current to the base thereof is temporarily terminated. Useless supply of base electrical current is prevented by performing control in this manner.
As described above, by performing ON/OFF control of the electrical current which is supplied to the base of the power transistor, the most suitable base electrical current is supplied, as an average, according to the time period width of the ON/OFF cycle. When the base electrical current is thus optimized, not only is it possible to shorten the storage time of the power transistor, but also it is possible to make the drive power supply circuit (in the figure, the insulating type DC-DC converter 2) smaller.
Moreover it is possible to control the load electrical current with the power transistor 1, since in the circuit shown in FIG. 6A the collector terminal of the power transistor 1 is connected to the power supply via the load.
FIG. 6B is a circuit diagram of an insulating type DC-DC converter according to the related art which is used in the circuit shown in FIG. 6A. The circuit shown in FIG. 6B is termed a forward type DC-DC converter. The operation of such a forward type DC-DC converter will now be explained.
When the switches S1 and S2 which are incorporated in this switching circuit are turned ON, electrical current is supplied to the primary side and to the secondary side of the transformer 3. The electrical current which flows in the secondary side flows through a choke coil L to a capacitor C and a drive circuit which is constituted by the load. The flow of energy at this time will now be explained. Energy is supplied to the transformer 3 from a direct current power supply 4. A portion of the energy which is supplied to the transformer 3 is accumulated in the core of the transformer 3, and excites the transformer 3. The remainder of the energy is transferred to the secondary side via the transformer 3 which has been excited. Of this energy which has been transferred to the secondary side, a portion is supplied to the load, while the remainder is accumulated in the choke coil L and the capacitor C.
After this, when the switches S1 and S2 are turned OFF, the excitation energy which has been accumulated in the core of the transformer 3 is returned to the direct current power supply 4 via diodes D1 and D2 on the input side (the side of the direct current power supply 4). The energy which has been accumulated in the choke coil L and the capacitor C is supplied to the load. In this manner it is possible to supply energy to the load, both when the switches S1 and S2 are ON and when they are OFF.
With the forward type DC-DC converter, it is necessary to accumulate the energy which is to be supplied to the load while the switches are OFF in the choke coil L. The maximum amount of energy per unit volume which can be accumulated in the choke coil L is determined by the characteristics of the magnetic material from which the choke coil L is manufactured, and therefore a large choke coil is required for accumulating a large amount of energy.
Generally, a control circuit is used which employs feedback, in order to stabilize the output electrical current from such a DC-DC converter, although this matter is not shown in the figure. Thus, the size and the cost of such a DC-DC converter are negatively impacted by the use of such a feedback circuit.
FIG. 6C is a circuit diagram of another insulating type DC-DC converter in the related art. The circuit shown in FIG. 6C is termed a flyback type DC-DC converter. The operation of such a flyback type DC-DC converter will now be explained.
When the switches S3 and S4 are turned ON, electrical current is supplied to the primary side of the transformer 5. At this time, energy from a direct current power supply 6 is supplied to the transformer 5, and energy is accumulated in the core of the transformer 5. However, in this case, the primary winding and the secondary winding of the transformer 5 are wound in opposite directions. Accordingly, no electrical current flows in the secondary side of the transformer 5, due to the provision of a diode D3. In other words, the energy which is accumulated in the core of the transformer 5 is not supplied to the secondary side of the transformer 5.
After this, when the switches S3 and S4 are turned OFF, the excitation energy which has been accumulated in the core of the transformer 5 flows via the diode D3 on the secondary side of the transformer 5 to a capacitor C and to the load.
With the above described flyback type DC-DC converter, since energy is not supplied to the secondary side of the transformer 5 while the switches S3 and S4 are ON, accordingly the energy which has been accumulated in the capacitor C is supplied to the load. Thus a large transformer is required, since it is necessary to accumulate energy in the core of the transformer while the switches are ON.
Furthermore, with this flyback type DC-DC converter, too, a feedback type control circuit is used in order to stabilize the output electrical current.
As shown in FIG. 6A, with a drive circuit for a power transistor according to the prior art, the most suitable pulse electrical current is obtained for driving the power transistor by supplying the output of the DC-DC converter to the base of the power transistor 1 via an ON gate circuit which can be selectively interrupted. Since as explained above the size of the DC-DC converter is determined by the amount of energy which it must supply, it is difficult to reduce the size of the circuit. Moreover, reduction in circuit size is also impeded by the requirement to provide a comparator or the like for generating the pulse electrical current.
Thus, the objective of the present invention is to provide a power supply circuit for driving a power transistor which is compact.
According to the present invention, the power supply circuit for driving a power transistor comprises: a direct current power supply; a transformer; a switching circuit which flows pulse electrical current through a primary winding of the transformer by connecting both ends of the primary winding of the transformer to both output ends of the direct current power supply in positive and negative directions alternately; a half wave rectification circuit which allows electrical current to flow in a secondary winding of the transformer when electrical current flows in the primary winding of the transformer in one or the other of the positive direction and the negative direction, or a full wave rectification circuit which allows electrical current to flow in the secondary winding of the transformer when electrical current flows in the primary winding of the transformer in either one of the positive direction and the negative direction; and an electrical current regulation circuit which is provided between an output of the half wave rectification circuit or the full wave rectification circuit and a control terminal of the power transistor, and whose resistance value changes according to a collector voltage of the power transistor.